Issue 21 came out this month; issue 22 will be out in November. From that point on, it will publish in odd-numbered months. It makes sense that they would do it this way, though the news about November surprised me at first.

Because the next issue isn't for 4 months, the final chapter of Mononoke Girls' Academy will be in Mononoke Girls' Academy volume 2, due out next month. (The penultimate chapter is in the issue of Yuri Hime that just came out.)

There is one chapter of Mononoke Girls' Academy in the latest Yuri Hime issue (which came out in Japan last month), and one more in the second Mononoke Girls' Academy collected volume (which is due out in Japan this month). That's it for the series.